The polar bear cubs Gregor and Aleut and their mother Vera explore their enclosure at the Tiergarten zoo in Nuremberg, Germany, 24 March 2011. The two cubs were born on 16 December 2010 and left their breeding burrow for the first time on 23 March 2010. David Ebener/CORBIS

July 21, 2011

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The future of polar bears, the preeminent "poster animal" for climate change, looks increasingly grim as their Arctic habitat changes. Particularly disturbing: Researchers are reporting that polar bear cubs, ill-equipped to tackle the latest challenges, are dying in increasingly high numbers as they struggle to keep up with their parents. Here, a brief guide:

What exactly did researchers find? By fitting adult bears with GPS collars, researchers have discovered that the animals are now going on marathon swims of unprecedented lengths — in one case, a mother polar bear swam for nine consecutive days, covering 426 miles (roughly the distance between Boston and Washington, D.C.). She survived, but lost 22 percent of her body weight. Another bear was tracked while swimming for more than 12 straight days.

Why are polar bears taking such epic swims? Polar bears evolved to hunt, give birth, and raise their young on pack ice in the Arctic. As temperatures rise, ice near the Arctic Circle is melting, so the bears have to swim farther to find what ice remains in the region. "Climate change is pulling the sea ice out from under polar bears' feet, forcing some to swim longer distances to find food and habitat," says Geoff York of World Wildlife Fund, as quoted by Reuters.

How do polar bear cubs fare on these swims? Not very well. They are smaller and leaner than their parents, and can't survive several days of swimming in rough open seas. The mother polar bear who swam for 426 miles had a young cub at the start of her journey, but lost him at some point during her nine days at sea. Scientists tracking one group of 68 bears reported that almost half of the cubs who were forced to swim with their mothers had died.

Is this the beginning of the end for polar bears? It doesn't look good. The amount of Arctic sea ice is at its lowest level since satellites began keeping records in 1979. That means polar bear cubs may have to keep making longer, deadlier swims. "We may need to find a new poster animal for climate change, something that really grabs our attention," says Bryan Walsh at TIME. "Perhaps human beings."